1. Field of the Invention
The present invention relates to a process for the treatment of surfaces of ferrous alloys to improve their frictional properties and resistance to wear and seizure, and to impart to them good accommodation capacity and good fatigue, by forming on the surfaces, by diffusion, intermetallic or semi-metallic compounds.
Note: by "accommodation capacity" should be understood the property possessed by surfaces to better distribute the loads pressing on them when originally they were not parallel and in conformity with the loading member surface.
2. Description of the Prior Art
At the present time various processes are known which make it possible to obtain coatings of this kind on the surface of a metal. This is in particular true of French Patent No. 1530 of the 6th May 1967 and its five certificates of addition, which describe processes for forming composite anti-friction layers by depositing a metal coating on a metal surface and then subjecting it to a suitable heat treatment. Two techniques have been proposed:
1ST TECHNIQUE
First, the deposition of successive layers of metals on the metal parts to be treated;
THEN DIFFUSION IN A PREFERABLY NEUTRAL OR REDUCING ATMOSPHERE, IN ACCORDANCE WITH A THERMAL CYCLE COMPRISING TWO SEPARATE PHASES: FIRST HEATING TO A TEMPERATURE AT LEAST 20.degree.C below the melting point of the most readily fusible of the metals present, then heating to a temperature between the said melting point and 800.degree.C.
2nd technique
Firstly, deposition on the part of a layer of an alloy of two or more metals,
THEN THE BAKING OF THE PART COATED IN THIS MANNER, OPTIONALLY IN A SINGLE STAGE.
In comparison with the first technique this second technique permits a shortening of the time during which the selected diffusion temperature is maintained.
After diffusion, the treated part presents in both cases, from inside to outside:
THE BASE METAL;
A LAYER OF RESIDUAL METAL;
A LAYER FORMED OF METALLIC COMPOUNDS OF A THICKNESS GREATER THAN 10 MICRONS AND OF A HARDNESS ABOVE 500 Vickers at 0.15 N;
a residual surface layer of a thickness less than 4 microns of one of the metals present.
The preceding technique is applicable to ferrous alloys and an example is described in the previously mentioned French Specification, in which the diffusion coating is produced from copper and tin, the temperature of the last stage of the heat treatment being indicated as equal to 600.degree.C. In this case the part presents after treatment, from inside to outside:
THE BASE FERROUS ALLOY,
A RESIDUAL LAYER OF COPPER,
A LAYER OF COPPER-TIN ALLOY FORMED OF VARIOUS PHASES AND OF A Vickers hardness between 500 and 800 at 0.15 Newton,
a surface layer of tin of a thickness of 1 micron.
These techniques for producing diffusion coatings based on copper and tin, when applied to ferrous alloys, nevertheless have disadvantages of which the principal are the following:
a. the layers formed have great porosity. When subjected to friction, this coating morphology results in rapid wear of the porous zone and a substantial progressive increase of the coefficient of friction; at a certain degree of wear there may even be epidermic sticking and seizure.
b. the presence of a layer of copper having poor mechanical characteristics under a layer of fragile compounds having good mechanical characteristics results in pronounced overall fragility of the composite layer, which prevents it from following any substantial deformations of the substrate without fracturing.
c. the fact that the hardness of the intermediate layer composed of metal compounds is higher than 500 Vickers entails very great fragility of this layer, which when subjected to friction may lead to flaking off.
In the above mentioned process, in the case of the copper and tin alloy originally deposited on the metal part, this alloy was obtained from known baths, for example baths based on cyanides, pyrophosphates, and fluoborates.
Nevertheless, these baths have the following main disadvantages:
a. the composition of the alloy deposited varies from one point to another on the treated part, and this variation is considerable. When the tin content of the mixed coating is lower than 17% by weight, the alloy, when subjected to friction becomes covered with copper, which in practice is incompatible with the usual mechanical parts, particularly steels, which have to rub against parts of treated ferrous alloys.
When the tin content exceeds 23% by weight, the fragility of the alloys formed becomes excessive and thus eliminates any possibility of accommodation of the composite layer formed. Furthermore, the parts coated with these alloys having a high tin content have their fatigue life reduced in proportions which may be as high as 25 or 30%.
b. These baths are chemically unstable, that is to say they rapidly change in the course of time and it is not possible for an alloy of a constant given composition to be deposited on successive parts, which, for a diffusion temperature suitable for a given composition of copper-tin alloy, results in partial fusion of the alloy in certain zones of the part, thereby irreversibly changing the surface states.
An object of the present invention is to provide a process by which it is possible, under industrial large-scale production conditions, to obtain on surfaces of ferrous alloys a coating having good frictional properties without seizing and with reduced wear, which is capable of following without fracturing any deformation, even substantial deformation, of its substrate, and which is not subject to surface creep.